The present invention relates in general to a projectable or push-out type writing instrument using an aqueous writing ink, and more particularly to a cartridge releasably and projectably secured in an instrument body or tubular casing, in which the cartridge contains therein an aqueous ink and a writing point.
A cartridge for the push-out type writing instrument, having a writing point and a filler impregnated with oleaginous ink is known, which is projectably held within a barrel or a casing of, for example, a ball point pen such that push-out operation permits the cartridge to resiliently project from its retracted position to an extended position for writing purposes. A writing instrument such as ball point pen incorporating such a projectable cartridge with oleaginous ink therein as described above is commercially available in the market. The oleaginous ink has a relatively high viscosity.
Many attempts and proposals have been made to provide a writing instrument using, in place of oleaginous ink, an aqueous ink which has been accepted widely as being capable of clear, smooth and fine writing, relative to the oleaginous ink. The attempts, however, have not been successfully matured into a practical application or actual project. One of the principal reasons for failure in practical application is based upon technological difficulties in producing a sealed (or air-tight) chamber in the casing of the writing instrument for preventing ink-shortage or ink-thinning due to drying by the air entering through a small air hole, which is generally provided for the purpose of smooth ink feeding action, at a front portion of the instrument. It is quite difficult to successfully provide such an air-tight chamber, and the present inventors have tried to provide the air hole at a rear end portion of the writing instrument.
However, repeated experiments have revealed some difficulties as set forth below.
(1) a water content in aqueous ink contained in an ink reservoir was greately decreased under a certain condition, for example, at the time of low humidity, to result in a highly increased viscosity of the aqueous ink in the ink reservoir, resulting in an unfavorable feeding of ink to the writing tip; and
(2) the air hole was made smaller so as to avoid an excessive decrease of water content in the aqueous ink, and then it was found that ink was not fed fully enough to the writing point, resulting in thinned and unreadable writing.